Large accelerations and tectonic setting of the May 1993 Shumagin Islands earthquake sequence

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Bulletin of the Seismological Society of America
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Variability of earthquake rupture can lead to large disparities between expected and observed ground motion. To better understand the range of ground motions generated by subduction zone earthquakes, we examine teleseismic and near-source strong-motion records from an interplate thrust earthquake sequence that occurred near the Shumagin Islands, Alaska Peninsula, in May 1993. The two largest events show source depths of 36 km (May 13) and 42 km (May 25) from inversion of teleseismic waveforms, which place them in the deepest part of the interplate thrust zone. This inference is verified by single-station epicenter estimates, derived from P-wave polarization and S-P times, and by the focal mechanisms. The 13 May 1993 mainshock (M(s) 6.9) shows peak accelerations of 0.7 to 1.4 m . sec(-2) within 75 km of the epicenter, consistent with scaling relations observed elsewhere. The largest aftershock, 25 May (M(w) 6.1), shows peak acceleration near 8 m . sec(-2) at a station 25 km away. The 8 m . sec(-2) acceleration is far larger than expected for an earthquake of this size and depth and is perhaps the largest recorded in Alaska. Simple explanations such as site effects and rupture directivity cannot fully account for the anomalous accelerations. The short rupture time of the 25 May event (similar to 2.8 sec) contributes to high accelerations but cannot explain the factor of 12 variability observed in peak acceleration near the epicenter. Unusual focusing effects and rupture patterns are suspected but cannot be confirmed. Whatever the cause, the unusually high accelerations demonstrate that deep and moderate-sized earthquakes can occasionally produce significant ground motions in subduction zones.


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